High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same composition by solid state polymerization. The low molecular weight polyesters which are used in such solid state polymerizations are prepared by conventional melt polymerizations. Solid state polymerization is generally considered advantageous in that the handling of high molecular weight ultra-high viscosity molten polymers during the polymerization phase is eliminated. Thermal degradation during the solid state portion of the polymerization is also essentially avoided.
In melt polymerizations, the major part of the reaction is transesterification due to the fact that thermal paralysis nullifies much of the esterification of carboxyl end groups. A much larger percentage of the reaction in a solid state polymerization is esterification. Thus, a high molecular weight can be obtained in a solid state polymerization with a lower carboxyl concentration than can be achieved in a melt polymerization.
The low molecular weight polyester prepolymers utilized in solid state polymerizations are generally in the form of pellets or chips. Such pellets can vary greatly in size; however, as a general rule, the smaller the size of the pellets of polyester prepolymer the faster the solid state polymerization with proceed. Such polyester prepolymers are generally converted from the amorphous to the crystalline state prior to solid state polymerization in order to raise their sticking temperature. This is done in order to keep the pellets or chips of polyester prepolymer from sticking together as a solid mass in the solid state polymerization reactor.
In the solid state polymerization of a polyester prepolymer the polymerization is carried out at an elevated temperature which is below the melting point of the polyester resin. Such polymerizations are normally conducted in the presence of a stream of inert gas or under a vacuum. Solid state polymerizations are normally conducted on a commercial basis in the presence of a stream of inert gas since it serves to remove volatile reaction products and helps to heat the polyester.